The present invention generally relates to an improved caliper in a disc braking system and, more particularly, to improvements in structure of the caliper housing for increased performance and prolonged service life of the braking system.
A caliper or caliper housing for use in a disc brake system comprises: a brake actuating hydraulic cylinder portion which includes a cylinder wall section having a cylinder bore open at one end thereof; a reaction portion which is positioned opposite to the cylinder portion axially of a disc rotor; and a bridging portion which extends between the opposing cylinder and reaction portions for connecting them. These three portions constitute an integral and substantially saddle-shaped structure which is adapted to straddle the disc rotor and a pair of brake pad assemblies disposed on opposite sides of, and in parallel to, the disc rotor, and which is adapted to be supported by a torque member movably along the axis of the rotor. Because the disc rotor and the caliper straddling the disc rotor are disposed in a limited open space formed inside the rim of a wheel to be braked, the maximum permissible diameter of the rotor is limited by the particular wall thickness of the bridging portion of the caliper. Therefore, it has been a long-felt but unsolved need to maximize the diameter of the disc rotor for enhancing the braking performance, and, more particularly, to increase the working area of the brake pad assemblies for prolonging an interval of maintenance of the braking system.
According to research of the inventors, the above requirement may be met by means of increasing the rigidity of the bridging portion, while reducing the wall thickness thereof, by forming the bridging portion so that it has a cross section, taken perpendicularly to the axis of the cylinder portion, which is of a relatively long circular arc configuration along the periphery of the rotor for forming an integral reinforcement structure or shell structure in combination with the cylinder portion and the reaction portion. Since, however, the cylinder portion of such type of caliper includes the cylinder wall section which extends radially inwardly of the rotor from the extremity of the bridging portion so that it faces the reaction portion and which has the cylinder bore open at one end thereof, it is unavoidable that the cylinder bore formed therein to roundness is deformed to an elliptical shape upon application of a reaction force transmitted thereto via the said wall section when a brake pad urging force is applied to the caliper upon actuation of the brake. Such deformation of the cylinder bore upon brake application will not only prevent normal operation of a piston slidably received in the bore in a fluid-tight manner, but also cause an abnormally rapid wearing of the piston and mating bore surfaces which results in fluid leaking from the cylinder.